Topoisomerase III enzymes are present only in a limited set of bacteria and their physiological role remains unclear. Here, we show that PbTopo III beta, a homologue of topoisomerase III encoded on the chromosome of Pectobacterium atrosepticum strain SCRI1043 (Pba SCRI1043), is involved in excision of HAI2, a discrete similar to 100 kb region, from the Pba SCRI1043 chromosome. HAI2 is a Pathogenicity Island (PAI) that encodes coronafacic acid (Cfa), a major virulence determinant required for infection of potato. PAIs are horizontally acquired genetic elements that in some instances are able to excise from the chromosome of their host cell to form a circular episome prior to transfer to a recipient bacterium. We demonstrate excision of HAI2 from the chromosome, a process that is independent of growth phase and that results in the production of a circular intermediate. Inactivation of PbTopo III beta causes a 103- to 104-fold increase in excision, leading to reduced fitness in vitro and a decrease in the virulence of Pba SCRI1043 on potato. These results suggest that PbTopo III beta is required for stable maintenance of HAI2 in the chromosome of Pba SCRI1043 and may control as yet unidentified genes involved in viability and virulence of Pba SCRI1043 on potato.

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Topoisomerase III enzymes are present only in a limited set of bacteria and their physiological role remains unclear. Here, we show that PbTopo III beta, a homologue of topoisomerase III encoded on the chromosome of Pectobacterium atrosepticum strain SCRI1043 (Pba SCRI1043), is involved in excision of HAI2, a discrete similar to 100 kb region, from the Pba SCRI1043 chromosome. HAI2 is a Pathogenicity Island (PAI) that encodes coronafacic acid (Cfa), a major virulence determinant required for infection of potato. PAIs are horizontally acquired genetic elements that in some instances are able to excise from the chromosome of their host cell to form a circular episome prior to transfer to a recipient bacterium. We demonstrate excision of HAI2 from the chromosome, a process that is independent of growth phase and that results in the production of a circular intermediate. Inactivation of PbTopo III beta causes a 103- to 104-fold increase in excision, leading to reduced fitness in vitro and a decrease in the virulence of Pba SCRI1043 on potato. These results suggest that PbTopo III beta is required for stable maintenance of HAI2 in the chromosome of Pba SCRI1043 and may control as yet unidentified genes involved in viability and virulence of Pba SCRI1043 on potato.